A dual-state emissive fluorescent probe for 2D and 3D intracellular picric acid sensing

Dual-state emissive (DSE) fluorophores, which exhibit efficient fluorescence in both solution and aggregated states, are attractive candidates for sensing and bioimaging applications. Herein, we report LY26, an aromatized imidazole-based DSE fluorophore for picric acid (PA) sensing in colloidal systems and intracellular environments. LY26 displayed pronounced solvatochromic fluorescence arising from intramolecular charge transfer (ICT). In tetrahydrofuran (THF)/water mixtures, LY26 underwent a characteristic twisted intramolecular charge transfer (TICT)-to-aggregation-induced emission (AIE) transition, exhibiting fluorescence quenching at intermediate water fractions followed by substantial fluorescence recovery in highly aggregated states. Strong fluorescence was maintained in solution (Φ_THF = 10.3%), nanoaggregates (Φ_{nanoaggregate} = 10.1% at f_w = 99%), and solid powder (Φ_solid = 32.1%), confirming its DSE characteristics. LY26 exhibited selective fluorescence quenching toward PA with a Stern-Volmer quenching constant (K_SV) of 5.27 × 10⁴ M^-1 and a limit of detection (LoD) of 2.98 μM. Spike-and-recovery experiments in real water samples afforded recoveries of 91.4-96.3%, demonstrating the practical applicability of LY26 for environmental PA detection. Encapsulation of LY26 within amphiphilic F127 polymers yielded LY26@F127 nanoparticles with excellent colloidal stability, photostability, low cytotoxicity, and efficient cellular uptake. Furthermore, concentration- and time-dependent intracellular PA sensing was successfully demonstrated in both 2D cell cultures and 3D cell-laden hydrogels. These findings establish LY26 as a promising DSE fluorophore for intracellular sensing and bioimaging in physiologically relevant cellular microenvironments.